An LED consists of a semiconductor junction, which emits light due to a current flowing through the junction. A white LED is typically made by using a blue or ultraviolet LED die, and adding a plastic coat to it, the coat containing a phosphor. The phosphor is used to convert the blue or ultraviolet light emitted by the LED die to a spectrum of light that more or less closely resembles white light or blackbody radiation.
At first sight, it would seem that white LEDs should make an excellent replacement for the traditional tungsten filament incandescent bulb. At equal power, they give far more light output than do incandescent bulbs, or, what is the same thing, they use much less power for equal light; and their operational life is orders of magnitude larger, namely, 10-100 thousand hours vs. 1-2 thousand hours.
However, LEDs have a number of drawbacks that have prevented them, so far, from being widely adopted as incandescent replacements. One of these is that, although LEDs require substantially less power for a given light output than do incandescent bulbs, it still takes many watts to generate adequate light for illumination. Whereas the tungsten filament in an incandescent bulb operates at a temperature of approximately 3000K, an LED cannot be allowed to get hotter than approximately 120° C., and some are limited to even lower maximum temperatures. The LED thus has a substantial heat problem: If operated in vacuum like an incandescent, or even in air, the LED would rapidly get too hot and fail. This has limited available LED bulbs to very low power (less than approximately 3 W), producing insufficient illumination for incandescent replacements.
One of the reasons that an LED is limited to such a low maximum temperature is due to the temperature characteristics of the phosphor rather than the LED die itself. Presently known phosphors, especially those in the red, tend to degrade quite rapidly at elevated temperatures. Once degradation has occurred, the white light output of the LED is reduced, thus ending the useful life of the LED and of the LED bulb.